1. Field of the Invention
The present invention relates to an overload control mechanism for use in a lever hoist, a chain block and a cargo binding machine and, particularly, to an overload control mechanism for assuring safety during an overloaded condition and preventing a failure in the machine.
2. Description of the Related Art
In cargo handling machines, such as a lever hoist or a chain block, maximum permissible loads, for example, 1 ton or 1.5 tons, are specified depending on the type of machine in order to assure safety during cargo handling operation. The machine is so designed as to present danger and damage as long as it is used below the maximum load.
FIGS. 4A, 4B and 5 show the construction of a conventional overload control mechanism in a lever hoist. A transmission gear (a) as a driving gear has, on its outer circumference, teeth (b) which are linked with a clutch and an operation handle (both not shown). Along with the clutch and handle, the transmission gear (a) rotates in whichever direction selected. The transmission gear (a) has an inner surface (c) tapered toward the its axis.
A friction disk (d) has a friction surface (e) having the same taper angle as the tapered surface (c) of the transmission gear (a), and a plurality of locking projections (f) on its inner edge. With the friction disk (d) in contact with the tapered surface (c), a friction force taking place therebetween causes the friction disk (d) to rotate integrally with the transmission gear (a). A support disk (g) has a threaded support shaft (h) that passes through the transmission gear (a) and friction disk (d). The support disk (g) has a plurality of notches (j) on its base portion from which the support shaft (h) is extended. The support disk (g) has along its center line a fixing shaft hole (k) through which is screwed a spindle (not shown) which causes a main gear to rotate through an appropriate reduction mechanism. When a cargo is raised or lowered, a rotary force acts on the spindle (with the support disk).
The support shaft (h) of the support disk (g) is inserted through the transmission gear (a) and the friction disk (d) and a nut (l) is tightened around the support shaft (h). The nut (l) holds together a conical spring washer (m), the friction disk (d) and the transmission gear (a) to the support disk (g) so that the friction surface (e) of the friction disk (d) is engaged with the tapered surface (c) of the transmission gear (a). The tightening force of the nut (l) adjusts the friction force between the tapered surface (c) and the friction surface (e) so that the maximum permissible load is set in relation to the rotary force exerted to the spindle.
More particularly, when the weight of a cargo is within the maximum load set (corresponding to the friction force), the transmission gear (a) is rotated by the operation of the handle, friction force taking place between the tapered surface (c) and the friction surface (e) exceeds the rotary force of the spindle, and the friction disk (d) is rotated integrally with the transmission gear (a). Furthermore, the friction disk (d) and the support disk (g) rotate integrally because the locking projections (f) are engaged with the notches (j). When the support disk (g) rotates, the spindle screwed into the support disk (g) rotates, permitting the main gear to raise or lower the cargo.
When the weight of a cargo exceeds the maximum load set, the rotary force occurring on the spindle becomes greater than the friction force. A slip takes place between the tapered surface (c) and the friction surface (e), and the friction disk (d) is unable to rotate even when the transmission gear (a) is rotated by the handle operation. The support disk (g), namely the spindle, stays still, unable to raise the cargo. Any failure or damage arising from the overload is thus prevented.
In the above control mechanism, the interfaces causing a friction force are only one existing between the friction surface (e) and the tapered surface (c). Repeated sliding actions for a long-term use or under an overload condition wear gradually the friction surface (e) and the tapered surface (c), thereby reducing the friction force and causing an error in the setting of the maximum load. The gap between the friction disk (d) and the nut (l) widens, weakening elasticity of the conical spring washer (m). For this reason, the machine is rendered unable to rotate even below the maximum load.
Although tightening the nut (l) may be a solution to the problem, leaving the setting job to ordinary users is not recommended from the standpoint of safety, and furthermore, standard design of the machine does not permit the user setting. As wear advances, the user may be forced to stop the use of the machine. As a result, the life of the machine is short.